Charlie and Jillian are both mercenaries, and they both tend to honor the letter of their contracts, while breaking the spirit of said contracts when it is in their interest to do so. One of Charlie's Rules reminds his Archons that by making trouble for their clients, they also make more business for Charlie.
As a former mercenary, Jillian probably understands Charlie's mercenary mindset better than any other Royal, which may be why she doesn't assume that Charlie and Stanley would become allies based on a common anti-Royal ideology. As a former Barbarian, she is also much more willing to take calculated risks than most other Royals, and so is willing to risk dealing with Charlie. She's confident enough to believe she has a good chance of benefiting from the association.
Jillian has long resented Charlie as her greatest competitor for mercenary contracts. At one time, she regarded Charlie as evil, especially because he had been trying to tempt her into signing a contract with the heroine buds. Jillian eventually agreed to sign the contract in exchange for saving Wanda. Charlie then violated the spirit, though probably not the letter, of their contract by destroying the section of Jillian's personality that manifested itself as the Jester. Charlie also erased most of what Jillian knew about him, including the fact that he had altered her personality. If Jillian ever learns what Charlie did, it is likely that she will become his implacable enemy. Jillian may have fought against Archons on occasion, before the Battle of Gobwin Knob, and may have sometimes fought alongside them, if her clients ever hired both her and Charlie. Given their sides' extremely different approaches to mercenary work (Charlie is very well known and works hard to further his reputation; Faq had a history of being veiled and took steps to have no local reputation at all), it is unclear how often they competed for the same contracts or were hired by opposing clients. However, Jillian worked far afield when Faq was still standing (to protect the side's anonymity), and Charlie deals with sides both near and distant, so the two of them are probably the most "worldly" participants in the conflict with Gobwin Knob, at least among non-Casters.
After the destruction of the first Royal Crown Coalition and the recreation of Faq, Charlie contacted her and they agreed to a deal. While we don't know its exact wording, it seems that he agreed to aid Jillian in return for her help in dealing with the threat posed by Wanda. Because all of the other members of the Royal Crown Coalition II refused to deal with him, Charlie needed Jillian badly. He helped build up Faq's power, presumably so as to be able to influence the decisions of the RCC II and create a viable proxy to attack Gobwin Knob, since he doesn't dare move against Parson directly. Fortunately, Jillian is willing to try Charlie's unconventional ideas, such as persuading the Western Giants to switch their loyalties from Jitterati to Faq. Charlie probably knows that Faq obtained much of its income from doing mercenary work, and may be concerned that it could eventually become a competitor once again, but given the magnitude of the crisis he faces, that possibility is likely to be of comparatively small importance.
Jillian never came to trust Charlie, but did talk to him often enough to make taking his calls habitual.
Charlie expected Jillian to croak Wanda, but Jillian tried instead to persuade Wanda to break with Stanley and join her. While having Jillian and Wanda join forces would not have been what Charlie or Don King had in mind when they decided to help Jillian, Jillian could argue that recruiting Wanda would have made Wanda less of a threat to Charlie, and would have done a great deal to weaken Gobwin Knob, which were the main goals they had in mind when they decided to help her.
Since Wanda refused to change sides, Jillian had Vanna end Gobwin Knob's turn. Though unwilling to croak Wanda herself, Jillian believed that Wanda's doom had been sufficiently insured to in itself fulfill the terms of her agreement with Charlie and end the threat Wanda poses. At this point, she decided to perform acts that would strengthen Faq at Gobwin Knob's expense, such as capturing Ansom and sacking some of Gobwin Knob's cities, while avoiding those acts that would weaken Faq, such as launching a frontal attack against Wanda's superior forces.
Jillian's sudden decision to change their agreed upon plan at Jetstone and not attack Wanda placed some stress on Jillian's alliance with Charlie. Charlie did not think that Jillian had ended the threat posed by Wanda, because he, unlike Jillian, knows that Parson has the ability to find a way that Wanda can escape the trap Jillian placed her in. He pretended to consider Jillian's revelation that Wanda does not hate Charlie and would probably be willing to work for him, but in actuality, he tried to have Ansom croaked out of anger at her failure to croak Wanda personally. As a result, Jillian wants nothing to do with a scheming manipulator like Charlie, while Charlie for his part has concluded that Jillian is too unreliable to work with. Charlie has told her to do as she likes, since she is now on her own.